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SA-213, Grade T22 60000 Hours Not Failures

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the
Conduit
A quarterly publication from
M&M Engineering Associates, Inc.
Vol. 11, No. 2
I N SI DE TH IS I SS UE:
Glossary of Boiler Tube Microstructures and Its
Use to Verify Oxide Thickness Temperature
Estimations
By Catherine A. Noble, P.E.
Senior Engineer
and
Ron Munson, P.E.
Corporate Engineer
The following is a summary of an article
written for CORROSION 2011, but with
new case studies. The full article can be
obtained at www.nace.org → Store →
Conference Papers.
INTRODUCTION
It is difficult to know what exact
temperature a boiler tube has been
exposed to over its lifetime. When
this information is needed for a tube
analysis, one possibility is to calculate
an equivalent tube metal temperature
Magnification: 1000X
Figure 1:
using the oxide scale thickness and
published data. However, this method
is not extremely reliable because of
the multiple factors that can affect /
disturb oxide growth.
M&M Engineering Associates, lnc. has
an ongoing research project that
examines microstructures of common
boiler materials as they are affected by
varying thermal exposures. With
these "standard” microstructures
available to compare with incoming
failures, one can increase the
confidence level of the temperature
exposure of the tube. The data can
also be used to confirm a tube's
temperature exposure that was
Dealloying and MIC in Heat
Exchangers
3
Central Africa Sugar Mill Steam
Turbine
5
Announcements
6
determined using the oxide thickness
method.
PROCEDURE
An aging schedule was established to
examine the materials at various levels
of heat exposure for assorted lengths
of time. The materials were exposed
to lower temperatures for longer
times and higher temperatures for
shorter times to more realistically
reflect actual life operations. Carbon
steels were aged at temperatures
ranging from 900°F to 1600°F with
exposure times ranging from 1000
hours for lower temperatures to 1
(Continued on page 2)
Magnification: 1000X
Photomicrographs show SA-210, Grade A-1 in the as-received condition and after 5 hours at 1300°F (704°C).
0.010 inch is equivalent to 0.254 mm. Sample was etched using Picral-Nital.
1
(Continued from page 1)
hour for higher temperatures. Alloy
steels were aged at temperatures
ranging from 1125°F to 1650°F with
exposure times ranging from 1500
hours for lower temperatures to 1
hour for higher temperatures.
Materials were also examined in the as
-received condition.
Materials that have already been
examined in the heat treatment study
include alloy grades from the
following ASTM specifications: A178,
A192, A210, A285, A515, and A213.
After heat treatment, each sample was
prepared for metallography and
documented at various magnifications.
Hardness was also measured.
HISTORY
One of the earliest uses of this
glossary was to quantify the extent
and duration of overheating of a coal
gasifier. The vessel had been in
service approximately ten years with
tube metal operating temperatures of
approximately 950°F. The vessel was
subjected to a hostile fire that visibly
overheated and distorted portions of
the vessel. The damage was thought
to be localized to one repairable
region, but the vessel owner wanted
to be certain that all damage was
repaired. Conversely, his insurance
company did not want to pay to
replace "good tubing." Metallographic
sections were removed from specific
areas of the vessel and a temperature
profile established. The tube
exposure temperatures were
estimated from the tubing
microstructures and a reasonable
repair plan was formulated. The
vessel owner and their insurance
carrier were pleased with the
outcome.
EXAMPLE
As an example of the use of the
glossary, SA-210, Grade A-11 is
Figure 2. Photomicrograph shows the condition of the T2 superheater tube
after 60,000 hours of service. Sample was etched using PicralNital.
Figure 3. Photomicrograph shows the oxide thickness (0.020 inch) on the
internal surface of a T2 superheater tube. Sample was etched
using Picral-Nital .
2
compared as-received and after heat
exposure to show the change in
microstructure. SA-210, Grade A-1 is
a medium-carbon steel commonly
used in boiler and superheater tubes
and has a maximum of 0.27 percent
carbon. The as-received
microstructure of the A-1 material
(shown in Figure 1) is a mixture of
ferrite and pearlite. This can be
compared to the microstructure after
five hours at 1300°F (704°C), also
shown in Figure 1. As the material
ages, the carbide morphology in the
pearlite breaks down. Initially, the
small discrete carbides on the
crystallographic planes agglomerate
into larger, more visible precipitates.
With additional time, these carbides
further degrade and release carbon
that migrates to the grain boundaries
and forms an intergranular carbide
precipitate.
line with the as-received condition of
T2. The microstructure shows that
the oxide thickness measurement was
not a good estimate of operating
temperature and the oxide was the
result of some other process. In this
case, some of the deposit likely
resulted from carry-over from the
steam circuit and was not formed in
situ.
CONCLUSIONS
Developing a glossary of
microstructures is a useful tool in
determining the temperature
exposure that an alloy has
experienced. Having a standard
reference condition makes it possible
to confirm operational data
concerning temperature profiles and
also provides a check for using oxide
thickness measurements to estimate
equivalent operating temperatures.
Additional alloys are being added to
this study, and service-aged samples
CASE STUDY
A superheater tube was received for a are also being added to the database.
metallurgical assessment. The tube
was made of SA-213, Grade T22 and
had been in service for approximately
60,000 hours. No failure was
REFERENCES
1. ASTM(1) A 210 (2010), “Standard Specificaobserved in this tube sample;
tion for Seamless Medium-Welded Carhowever, several failures occurred in
bon Steel and Carbon-Manganese Steel
this particular superheater. The
Boiler and Superheater Tubes” (West
Conshohocken, PA: ASTM).
microstructure of the tube is shown in
2. ASTM A 213 (2010), “Standard Specification
Figure 2 and was typical of like-new
for Seamless Ferritic and Austenitic AlloyT2 material with bainite colonies and
Steel Boiler, Superheater, and HeatExchanger Tubes” (West Conshohocken,
dispersed carbides. The internal oxide
PA: ASTM).
thickness was measured as 0.020 inch
3. R. Viswanathan, Damage Mechanisms and Life
(0.051 cm), which is unreasonably
Assessment of High-Temperature Components (Metals Park, OH: ASM Internalarge for a tube with so few operating
tional, 1989), p. 229.
hours (Figure 3). Using internal
4. S.R. Paterson, T.W. Rettig, “Remaining Life
oxidation growth rate data available
Estimation of Boiler Pressure Parts-2 ¼
for chromium-molybdenum steels3, 4,
Cr-1Mo Superheater and Reheater
Tubes,” Project RP 2253-5, Final Report
this translates to an equivalent tube
(Palo Alto, CA: Electric Power Research
temperature between 1200°F (649°C)
Institute, 1987).
and 1300°F (704°C). However, the
(1) ASTM International, 100 Barr Harbor Dr.,
West Conshohocken, PA 19428-2959.
microstructure in Figure 2 is not
consistent with this calculated
temperature exposure and is more in-
Dealloying/Selective
Leaching/of Copper
Alloys due to
Microbiologically
Influenced Corrosion
(MIC) in Heat
Exchangers
By Spencer Rex
Mechanical Engineer
and
David Daniels
Principal Scientist
Copper alloys have been used
extensively in a number of industrial
water cooled heat exchangers because
of their excellent thermal conductivity
properties. A disadvantage of these
materials is that they are relatively soft
and will corrode for a variety of
reasons. Copper alloys that have been
used in industrial heat exchanger
applications include, 66.5Ni/31.5Cu/Fe
(Alloy 400 or N04400), 90Cu/10Ni
(C70600), 70Cu/30Ni(C71500), and
arsenical Admiralty Brass (44300).
Dealloying can occur in copper alloy
heat exchanger tubing caused by
microbiologically induced corrosion
(MIC), particularly in Admiralty Brass.
Bacteria, fungi, and algae all can be
found in cooling water. Cooling
towers act as large air filters, pulling in
outside air that contains dust and
bacterial spores into the tower where
they are mixed with the cooling water.
Many heat exchangers provide
consistent warm temperatures and a
continuous supply of aerated water-an excellent place for all types of
microbiological species to take hold.
When microbes settle on a surface
they set up a protective organic layer
called a biofilm, underneath which the
microbes live protected from
(Continued on page 4)
3
dot map of the dealloyed area
generated by energy dispersive X-ray
spectroscopy (EDS). The dot map
highlights the presence of a single
element in the matrix, in this case
nickel. Note the relative absence of
nickel in the area where dealloying has
taken place. Established biofilms are
thicker and better at developing types
of bacteria that generate strong
corrosive acids. While there is
nothing that can completely prevent
biofilm formation, controlling biofilm
development with biocides minimizes
the potential for dealloying. In many
cases, copper alloys are being replaced
Figure 1. Biofilm
by stainless steel or titanium in
(Continued from page 3)
condensers which are more corrosion
resistant but have a lower thermal
noble element in the alloy as the
environmental changes including
anode, and the water in the biofilm is conductivity than copper alloys, so
biocide addition (Figure 1).
there is a trade-off. Also, stainless
the electrolyte.
Underneath the biofilm and adjacent
steel tubes are also susceptible to MIC
The result of dealloying is a metal
to the metal surface, waste products
matrix where the less noble element attack.
from the bacterial respiration can
accumulate. They are typically more in the alloy has oxidized, causing it to To protect against dealloying of
copper alloys by microbiologically
become soluble and dissolve. The
acidic than the bulk cooling water.
remaining copper metal has a spongy” influenced corrosion, the proper
These conditions are conducive to
appearance and the metal has changed selection and concentration of
dealloying of copper alloys.
biocides must be used control
Dealloying will occur underneath the from brass color to copper color
biofilms. Because biocides do not
(Figure 2). The copper sponge has
biofilm when all the components of
completely remove the biofilm,
very little remaining structural
the electrochemical/galvanic cell
strength. This can lead to a brittle and periodic mechanical removal of the
(cathode, anode and electrolyte) are
in electrical contact with one another. sometimes catastrophic failure of the biofilm (e.g., nylon bristle brushes or
tube. The extent of the dealloying in scrapers) and any debris in the tube,
In the case of dealloying of copper
is critical.
alloys, the copper acts as the cathode, this condenser tube can be seen in
Figure 3. Figure 4 shows an elemental
the zinc or nickel or any other less
Copper Nickel Alloy
Dealloyed Copper
Figure 2. Pure copper remains after the
nickel has been removed.
Figure 3. The area where dealloying has Figure 4. Absence of of Nickel (Blue) as
taken place is inside the red box. This
seen in the EDS dotmap where the dealsame area is shown in Figure 4.
loying has occurred.
4
M&M Engineering—
Helping to Keep the
Lights On and Life
Sweet in Central
Africa
By David Daniels
Principal Scientist
A few years ago a sugar mill in Central
Africa decided to upgrade a number of
smaller bagasse boilers with a larger
higher pressure (45 bar) boiler capable
of not only generating the required
mill steam but also a reliable source of
electricity for the mill and the
surrounding area. Up to that point,
electrical power in the area had been
sporadic. Power outages were
common for one or two hours nearly
every evening.
The mill purchased a 10 MW steam
turbine with extraction points to
supply the mill process and a small
condenser for the remaining steam.
After a short period of operation, the
steam turbine developed performance
issues. When the mill opened the
steam turbine, they found it covered
in white and rust colored deposits.
Analysis showed that the deposits
were predominantly silica and caustic.
The turbine manufacturer
recommended M&M Engineering to
the mill. We visited the site, evaluated
the water and steam chemistry
program, determined the probable
causes of the deposits and developed a
plan for minimizing steam
contamination in the future. The
recommendations included changes in
the boiler operation, chemical
treatment, monitoring, and water
pretreatment practices. We provided
a new set of chemistry guidelines and
provided training to the operators and
lab personnel on testing, water
treatment equipment, and handling
BEFORE
chemical upsets. The
mill took action and
made many of the
recommended
changes before the
next campaign.
A year later, M&M
Engineering was asked
to return during the
mill outage where
they were again
removing the turbine
cover for inspection.
We found the turbine
was far cleaner with
Figure 1. After only a short period of operation, the
minimal deposits.
turbine became contaminated with deposits.
While there, we were
able to find other
AFTER
ways for the plant to
improve the reliability
and reduce the costs
associated with their
steam cycle.
Sugar mills are net
water producers as
water from the cane is
removed by a series of
evaporators that
concentrate the syrup
to the point that it
crystallizes. The
various streams of
condensed vapor from Figure 2. The mill improved operating practices and
the evaporators are
chemical monitoring that made a tremendous
improvement in the turbine’s appearance.
known as clean and
dirty condensate
resulted in far longer runs on the ion
depending on the propensity for the
exchange demineralizer than they had
condensate to be contaminated not
only with sugar, but also organic acids experienced previously using a very
hard well water. We also worked
and alcohols that are part of the
with the mill to revised the cooling
natural fermentation process.
tower chemistry program to allow
M&M Engineering worked with the
mill to find appropriate ways to reuse them to use a source of “dirty
condensate” as partial makeup to their
water sources that otherwise were
going to waste. The mill had a water cooling tower. This also reduced the
mill’s use of a very highly mineralized
pretreatment system that could be
well water for cooling tower makeup.
used to collect a relatively clean
source of condensate known as V1 for This allows the cooling tower to
operate at higher cycles of
preparing boiler feedwater. This
5
(Continued from page 5)
concentration with better control of
scaling.
M&M Engineering continues to make
annual visits to the mill to check on
the boiler and steam chemistry, water
pretreatment, and cooling water
chemistry.
Now that the steam turbine issues
have been resolved, the lights at the
mill and in the surrounding area stay
on constantly. Reducing the amount
of well water used helps the mill and
people in the area.
Contact the Authors
Catherine Noble, P.E.
Senior Engineer
512-407-3771
catherine_noble@mmengineering.com
David Daniels
Principal Scientist
512-407-3752
david_daniels@mmengineering.com
New Employees Join M&M Engineering
Anna Gentry joined
M&M Engineering
Associates, Inc. in
May 2011 as a
Mechanical
Engineer. She has
always been
interested in the way things work and taking
them apart to find out just how. Anna holds a
Master of Science in Mechanical Engineering
from Texas Tech University with an emphasis
in Mechanics and Materials. In graduate school
she also explored the microscopic world while
studying electron microscopy. Prior to joining
M&M Engineering, Anna worked as a Stress
Analysis Engineer helping to design, model,
test and redesign the piping of a Naptha
Hydrotreater Unit for an all new refinery to
be built in South America. She also has
experience in boots and a hardhat as a Field
Engineer constructing full containment LNG
tanks on the Gulf Coast of Texas. When not
at work Anna likes to solve puzzles of the
mathematical and jigsaw varieties, dance the
Lindy Hop, enjoy the Texas Hill Country and
crochet.
Candice Chastain
joined M&M
Engineering
Associates, Inc. in
February 2011 as an
Administrative
Assistant . Candice
provides an
administrative backbone for the engineers,
scientists and staff of M&M Engineering. She
ensures that every proposal, report and
presentation that goes out under the M&M
Engineering name has the proper quality and
polish. She also manages incoming calls for her
coworkers, performs electronic and hardcopy
archiving, handles multiple incoming and
outgoing mail and freight services, plans events
and conferences, and keeps the office up to
date and organized. Before joining M&M
Engineering, Candice lived in Fort Worth,
Texas, were she was a small business owner,
executive administrative assistant and an office
manager several times over. She holds an
Associate of Science Degree from North
Central Texas College. Since moving down
from the Fort Worth area, Candice has
enjoyed gardening, cooking, wine and
exploring Austin with her doggies.
Seminars & Workshops
Ron Munson attended the International Gas Turbine Institute
Conference in Vancouver, Canada June 6-10, 2011.
Jon McFarlen attended the ASME Power 2011 Conference in
Denver, CO (July 12-14) and was a panelist for the “Aging of
Combined Cycle Power Plants” discussion session.
TAPPI, the Technical Association of the Pulp and Paper Industry, had the PEERS conference in
Portland, Oregon this year October 2- 5, 2011. Ron Lansing attended the TAPPI (Technical
Association of the Pulp and Paper Industry ) Engineering Conference. M&M Engineering regularly
participates in this conference and the Engineering Committees Corrosion and Materials subcommittee and task groups.
Jon McFarlen gave a presentation entitled How to Assess the Health of High-Energy Piping Systems on October 31, 2011
at the Combined Cycle Users Group 2011 meeting in San Antonio, Texas.
Henry Kight attended ASM International’s training on Metallography for Failure Analysis in Cleveland, Ohio August 22-26, 2011
Please visit us at www.mmengineering.com for additional information regarding conferences and events.
6
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The
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the Conduit
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All of us at M&M Engineering would like to take this opportunity to say
thank you for contributing to our success and growth this past year.
We sincerely hope that the upcoming year is filled with much happiness
and prosperity for you and your family.
Have a joyous holiday season.
8
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